From the Earth to the whale: Opportunistic use of ocean-bottom seismometers to track fin whales

Monitoring baleen whales often involves significant costs, making the use of opportunistic data from instruments deployed for other purposes an attractive and cost-effective alternative. However, such data typically require adjustments to standard analytical approaches. The CORTADO project (Combining global OBS and CTBTO recordings to estimate abundance and density of fin and blue whales) uses data from two types of bottom-deployed sensors to develop and implement methods for tracking and estimating the density of fin (Balaenoptera physalus) and blue (B. musculus) whales. While Ocean Bottom Seismometers (OBS) and hydroacoustic data from the Comprehensive Test Ban Treaty Organization (CTBTO) have proven to be valuable in prior studies, tools for routine application of these datasets remain limited. The primary objective of CORTADO is to develop a set of software tools and training resources to facilitate the analysis of large, historical datasets of baleen whales. This presentation focuses on phase 1 of CORTADO, showcasing the workflow and comparing two single-station ranging techniques for fin whales: the particle velocity method and the multipath method. Using OBS datasets from six deployment areas—three in the Pacific Ocean (Marianas Trench, Hawaii, and Oregon OOI) and three in the North Atlantic Ocean (ENAM, Azores, and Gulf of Cadiz)—ranges to 20-Hz fin whale calls were estimated and averaged into 1-minute bins. Differences between the two ranging methods were further assessed with a Generalized Additive Mixed Model (GAMM) to account for the different tracks and areas. Results indicated that the multipath method achieved larger range estimates, exceeding 15 km, while the particle velocity method was limited by a site-specific validity range influenced by OBS depth and propagation properties. The multipath method performed best in sedimented areas with identifiable multipath arrivals, while the particle velocity method was more effective in deeper, softly sedimented regions. Both methods showed challenges for complex bathymetry, complex calling behaviour, and chorusing. On average, multipath estimates were 557 m greater than particle velocity estimates, but variability was highly dependent on site and track conditions, with sediment type being a key factor. These findings provide critical insights into the performance of single-station ranging techniques and their application, contributing to the broader usability of opportunistic datasets for marine conservation. This work is supported by the Portuguese Fundação para a Ciência e Tecnologia, FCT, I.P./MCTES through national funds (PIDDAC): UID/50019/2025, UIDB/50019/2020 (https://doi.org/10.54499/UIDB/50019/2020), LA/P/0068/2020 (https://doi.org/10.54499/LA/P/0068/2020) and UIDB/00006/2020 (https://doi.org/10.54499/UIDB/00006/2020). It is also supported by US Navy Living Marine Resources program award N39430-21-C-2208 and N00014-21-1-2564, and EC project SUBMERSE project HORIZON-INFRA-2022-TECH-01-101095055.